The present bright source attenuating device may be used at the input of an image intensifier tube to reduce glare and halation from bright light sources that are present in the field-of-view of the image intensifier. This bright source attenuating device is however not intended to be limited to that function. As examples, the present bright source attenuating device may be used as erasable photographic film, as a memory device, in binoculars for monoculars, or in television cameras that are used in high light contrast areas such as at a football game.
The problem of glare was never solved satisfactorily in early design image intensifier tubes, known as first generation image intensifiers, but the glare was limited to a smaller area in the second generation tubes through the use of microchannel plates. The halo problem is still present even in the third generation image intensifier tubes and the image around a bright light is washed out by this halo. The extent of the glare and the diameter of the halo is much greater in the wide gap diode, a third generation tube without a microchannel plate and having a wide gap between the photocathode layer and the phosphor screen. Other electrochromic and photoconductive devices have been patented but none was designed to automatically and selectively attenuate bright spots, especially in the near infrared spectrum, for an imaging device. These previous patents were drawn toward visible display or image recording purposes and were not suitable for image intensifier protection. Three patents that are believed to be exemplary of similar prior art light attenuating devices are U.S. Pat. Nos. 3,373,091; 3,521,941 and 3,589,896.
As far as the inventor knows, no devices of the present type that limit bright sources are presently being used. Internal improvements in the image intensifier tubes, such as antireflective coatings and microchannel plates, have reduced the glare but have not solved the problem. These improvements however do not help in situations where a bright spot activates the current limiting function of the tube power supply and reduces the voltage applied to the tube causing degraded performance over the whole image plane. The present bright source attenuating device responds to visible bright lights and also attenuates these bright lights over the spectral regions where image intensifier tubes are sensitive, including the near-infrared spectral region, and may therefore attenuate a bright source prior to its entry into the tube.